Plasma display panels, or gas discharge panels, are well known in the art and, in general, comprise a structure including a pair of substrates respectively supporting thereon column and row electrodes each coated with a dielectric layer such as a glass material and disposed in parallel spaced relation to define a gap therebetween in which an ionizable gas is sealed. Moreover, the substrates are arranged such that the electrodes are disposed in orthogonal relation to one another thereby defining points of intersection which in turn define discharge cells at which selective discharges may be established to provide a desired storage or display function. It is also known to operate such panels with AC voltages and particularly to provide a write voltage which exceeds the firing voltage at a given discharge point, as defined by a selected column and row electrode, thereby to produce a discharge at a selected cell. The discharge at the selected cell can be continuously "sustained" by applying an alternating sustain voltage (which, by itself is insufficient to initiate a discharge). This technique relies upon the wall charges which are generated on the dielectric layers of the substrates which, in conjunction with the sustain voltage, operate to maintain discharges.
Details of the structure and operation of such gas discharge panels or plasma displays are set forth in U.S. Pat. No. 3,559,190 issued Jan. 26. 1971 to Donald L. Bitzer, et al.
In the past two decades, AC plasma displays have found widespread use due to their excellent optical qualities and flat panel characteristics. These qualities have made plasma displays a leader in the flat-panel display market. However, plasma panels have gained only a small portion of their potential market because of competition from lower cost CRT products.
The expense of the display electronics, not the display itself, is the most significant cost factor in plasma displays. Because of the matrix addressing schemes used, a separate voltage driver is required for each display electrode. Therefore, a typical 512.times.512 pixel display requires a total of 1024 electronic drivers and connections which add considerable bulk and cost to the final product. To combat this problem, many techniques have been suggested to reduce the number of electrodes that need separate circuit drivers. The most popular methods have resulted in shift and logic panels. However, the use of these products have been limited by inherently slow panel update rates and by the expensive manufacturing processes required to fabricate the complex panels.
In a shift panel, all of the display information is typically entered along the vertical edge of the panel and then shifted across it. This method requires the same number of drivers along the vertical edge but only a small number along the horizontal edge (usually less than ten). Thus almost a two to one reduction in circuit requirements can be achieved.
The state of a particular cell within a shift panel, whether on or off, is shifted to the next adjacent cell by using either priming coupling or wall-charge coupling. The first method, priming coupling, uses priming particles produced by a nearby on cell to reduce the firing voltage at the next cell location in the shift sequence. A voltage is applied to the next cell in the sequence so that it will turn on only if it receives sufficient priming from its neighboring cell. Thus information is shifted from cell to cell across the entire display.
The priming technique relies on the presence of priming particles generated at an addressed cell location to effect one of its surrounding display pixels in such a way that allows it to be written or erased. Therefore, to use the priming technique in other than a shift mode, the proximity of the selected pixel to the selected address cell must be the controlling effect that allows it to be selected from the thousands of other display pixels receiving the same voltage signals.
The priming particles produced from a discharge are somewhat locally constrained. Put simply, electrons, ions, metastables, and photons produced in a gas-discharge can serve as agents in priming other nearby cells. The electrons, ions, and metastables are confined to affect only nearby cells because of their interactions with the electric fields and reactions with other gas-discharge phenomenon. The photons, however, are unhindered by any of the events in the gas volume and can effectively prime cells at some distance. Thus, the main drawback of priming is that other cells at a distance are caused to be "primed" in addition to the selected cells, resulting in spurious discharges - an undesirable result.
Wall-charge coupling, as its name implies, uses the actual transfer of charge from one cell to the next to accomplish the shifting. In this scheme, a cell will only fire if it receives a transfer of electrons produced when proper voltages are applied to a neighboring cell, one of whose electrodes is common to both cells. In such case, the common electrode acquires a wall charge which influences the discharge state of the next cell in line. Such types of devices are described by W. E. Coleman et al, in "Device Characteristics of the Plasma Charge Transfer Shift Display," IEEE Transactions on Electron Devices, Vol. ED-28, pp. 673-679, June, 1981.
A somewhat different type of shift panel is described in U.S. Pats. Nos. 4,430,601 and 4,328,489 to Peter D. T. Ngo. The shift panels disclosed therein include both display columns and "transfer" columns. When a transfer of data is described, an excitation pulse is applied to the display column and a priming pulse to the transfer column. The plasma discharge created by the excitation pulse is said to be transported to the transfer column by the action of the priming pulse causing it to switch to the ON state. Careful analysis of the Ngo structure shows, however, that the plasma discharge is actually forced to migrate to an area whose residual wall voltages are invariably more negative than the wall voltage at the display site where the plasma originated. In essence the plasma is forced to travel "uphill" and the resulting panel operation has been shown to have poor operational margins.
Shift panels suffer from a number of other disadvantages such as slow panel update rates; no random access ability; and poor yields due to the fact that one defective pixel will cause the loss of an entire line.
Another approach to reducing circuit requirements involves the use of gas-discharge logic. One such scheme is described in Schermerhorn U.S. Pat. No. 3,925,703 and by Schermerhorn and Miller in "Discharge Logic Drive Schemes", IEEE Transactions in Electron Devices, Vol. ED-22, No. 9, pp. 669-673, Sept., 1975. In that arrangement, a display pixel is defined by the intersection of two pairs of orthogonal electrodes. The electrodes are grouped in such a way as to give each pixel four distinct inputs. A particular pixel may be erased only when all four of its inputs are selected. This gas-discharge action is functionally equivalent to a four input AND gate and employs a plasma coupling or spreading technique.
Schermerhorn's logic technique results in a rather dramatic reduction in line driver requirements. In a typical 512.times.512 panel, 48 lines per axis or a total of 96 lines are required. This represents an order of magnitude in savings over the normal 1024 required drivers. However, the savings in circuit costs are offset by the increase in panel manufacturing cost. Crossovers increase in number and area with increasing panel size and add expensive fabrication cost to the panel. Furthermore, the sustain signal is applied to all drive lines resulting in the sustain driver being a very high current device with a corresponding requirement that it exhibit a low impedance, a particularly difficult and expensive set of criteria to meet.
One method to reduce the electronics cost has been the development of plasma AND gates as described in copending U.S. patent application Ser. No. 462,029, assigned to the same assignee as is this application. Such AND gates can be integrated into the display panel itself, increasing its cost only slightly, while reducing electronics requirements by more than an order of magnitude. These devices have the capability of driving the capacitive load of an address electrode in a standard AC plasma panel; however, it has been found that the 20 mA sustain discharge current that is required when all the pixels along an electrode are discharging causes an undesirable voltage droop which is likely to reduce the panel operating margin excessively.
Attempts have been made to separate the sustain operation from the address circuitry. In this regard Andoh, et al. U.S. Pat. No. 4,044,349 employs 4 discharge site pixel locations and applies enhanced X and Y sustain-type pulses to one discharge site and a standard sustain signal to another site. When the addressed site discharges, particles from the discharge affect the other three discharge sites and reduce their firing levels. As a result the subsequent application of a sustain signal to the pixel site turns on the pixel site as well as the other sites. Andoh's write technique, which employs sustain-type signals, is very slow, and the use of the priming particles to influence adjacent discharge sites leads to poor operational voltage margins for the panel.